ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-10-10917-2010 Different photolysis kinetics at the surface of frozen freshwater vs. frozen salt solutions Kahan T. F. 1 3 Kwamena N.-O. A. 1 Donaldson D. J. 1 2 Department of Chemistry, University of Toronto, 80 Saint George Street, Toronto, M5S 3H6 Ontario, Canada Department of Physical and Environmental Sciences, University of Toronto at Scarborough, Canada currently at: Department of Chemistry, University of California Irvine, Irvine, California, 92697, USA 22 11 2010 10 22 10917 10922 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/10/10917/2010/acp-10-10917-2010.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/10/10917/2010/acp-10-10917-2010.pdf

Reactions at air-ice interfaces can proceed at very different rates than those in aqueous solution, due to the unique disordered region at the ice surface known as the quasi-liquid layer (QLL) . The physical and chemical nature of the surfacial region of ice is greatly affected by solutes such as sodium halide salts. In this work, we studied the effects of sodium chloride and sodium bromide on the photolysis kinetics of harmine, an aromatic organic compound, in aqueous solution and at the surface of frozen salt solutions above the eutectic temperature. In common with other aromatic organic compounds we have studied, harmine photolysis is much faster on ice surfaces than in aqueous solution, but the presence of NaCl or NaBr – which does not affect photolysis kinetics in solution – reduces the photolysis rate on ice. The rate decreases monotonically with increasing salt concentration; at the concentrations found in seawater, harmine photolysis at the surface of frozen salt solutions proceeds at the same rate as in aqueous solution. These results suggest that the brine excluded to the surfaces of frozen salt solutions is a true aqueous solution, and so it may be possible to use aqueous-phase kinetics to predict photolysis rates at sea ice surfaces. This is in marked contrast to the result at the surface of frozen freshwater samples, where reaction kinetics are often not well-described by aqueous-phase processes.

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